Seem to be only two Niviventer species there: N. tenaster and N. fulvescens. The former generally fits although it’s supposed to have an orangish line on the flank; the latter is usually yellowish or reddish brown with bicolored tail. But why do you think it’s a Niviventer and not a Rattus?

Simply because it didn’t seem ‘quite right’ for a Rattus and because it looked quite good for a Niviventer in the Francis guide but I have no experience for Niviventers and I am more than happy to be corrected.